Dual thermodynamic cycle cryogenically fueled systems

ABSTRACT

Systems and methods for converting thermal energy, such as solar energy, from a localized thermal energy source to another form of energy or work comprise dual thermodynamic cycle systems that utilize the liquid-to-gas phase transitions of a cryogenic fluid such as liquid nitrogen and a working fluid such as sulfur hexafluoride to drive prime movers. Heat transfer between the fluids as they undergo the phase transitions is used to increase the energy in the system and its work output, and improve system efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent Application No. 60/735,056, filed Nov. 8, 2005, and U.S. provisional patent Application No. 60/737,682, filed Nov. 17, 2005, the disclosures of which are hereby incorporated by reference herein.

TECHNICAL FIELD

The invention relates generally to methods and apparatus utilizing cryogenic fluids, and more particularly to methods and apparatus for utilizing cryogenic fluids for thermal energy conversion and to operate prime movers.

BACKGROUND OF THE INVENTION

Systems utilizing cryogenic fluids to operate prime movers (e.g., engines, turbines, motors, pumps, generators, and equivalents) to produce various forms of energy have been investigated as environmentally clean sources of energy. A cryogenic automobile is a zero-emission vehicle, and one example of such a clean energy system. It operates on the thermodynamic potential between the ambient atmosphere and a reservoir of liquid nitrogen. One way to utilize that potential is through an open Rankine cycle. Liquid nitrogen is drawn from a tank at the system pressure, then vaporized and superheated in a two-stage heat exchange system. The resulting high pressure, near-ambient temperature gas is injected into a quasi-isothermal expander that produces the system's motive work. The spent, low-pressure gas is exhausted back to the atmosphere.

Although liquid nitrogen powered engines useful for example in an automobile have been studied in the past, there have been problems with such engines, including implementing a quasi-isothermal expander and a frost-free liquid nitrogen heat exchange system. There are many thermodynamic cycles available for utilizing the thermal potential of liquid nitrogen. These thermodynamic cycles range from the Brayton cycle, to two- and even three-fluid topping cycles, to employing a hydrocarbon-fueled boiler for superheating beyond atmospheric temperatures. The easiest system to implement, and the one studied at the University of Washington, uses an open Rankine cycle. In this system, the ambient temperature was used to boil the liquid nitrogen and to raise the pressure in the high pressure side of the engine to on the order of 30-50 bar. No work was extracted from the cryogenic fluid in the fluid-to-gas transformation. All work was dissipated into the ambient around the engine.

There are several problems with using cryogenic fluids, including that cryogenic fluids need very effective thermal insulation, have significant problems with ice condensation within an engine from the cold temperature fluids used, and are extremely inefficient for the production of mechanical power from the phase change of the cryogenic fluid to an expanding gas. The extremely inefficient production of mechanical power from the phase change of the cryogenic fluid to an expanding gas is the biggest problem with existing technology.

Using phase change in a cryogenic fluid has additional serious problems. Conventional systems merely dump the heat from the cold cryogenic fluid at atmospheric ambient temperatures, and do not fully utilize the thermodynamic possibilities available from the cryogenic fluid. Some systems use nitrogen gas to preheat the cryogenic fluid before it enters the heat exchanger, but this only provides a minor improvement in the overall efficiency of the energy conversion in the engine.

Conventional thermal power engines utilizing cryogenic fluids suffer from a serious limitation, which is low efficiency in the energy conversion. A need exists for a higher efficiency system for thermal energy utilization in general, and especially for solar energy, which does not require expensive equipment, materials, and maintenance. What is needed is a relatively stable and non-degradable thermal energy utilization apparatus and method for long-term operation to utilize thermal energy that is more efficient and that overcomes the shortcomings described above.

SUMMARY OF THE INVENTION

The present invention affords a system and method to utilize ambient thermal energy, including solar energy, geothermal energy, waste-heat energy, bio-mass combustion energy, and other equivalent types of energy, and for using cryogenic fluids.

In a first aspect, the invention affords a method of converting thermal energy that includes transferring thermal energy into a cryogenic fluid from a working fluid to expand a portion of the cryogenic fluid and generate a first gas and condense the working fluid to a liquid; expanding at least a portion of the working fluid by transferring thermal energy to the working fluid to create a second gas; utilizing the second gas to operate an energy conversion apparatus; and transferring additional thermal energy to the first gas from second gas exhausted from the energy conversion apparatus to substantially condense the second gas into the working fluid, and utilizing the first gas with increased energy to operate another energy conversion apparatus.

In another aspect, the invention affords a method of converting thermal energy by expanding a portion of a working fluid in a first system that operates according to a first thermodynamic cycle to generate a first gas for operating energy conversion apparatus; expanding a portion of a cryogenic fluid in a second system operating according to another thermodynamic cycle to generate a second gas for operating other energy conversion apparatus. The two systems are coupled together, and the method further involves transferring additional heat from the first gas to the second gas to increase the energy of the second gas.

In a further aspect, the energy conversion apparatus comprises a prime mover, and the inventive method expands a portion of a working fluid in a first container to create a first gas that operates a first prime mover. Gas is exhausted from the first prime mover into a second container immersed in a cryogenic fluid in a third container to expand a portion of the cryogenic fluid and create a second gas that operates a second prime mover. The first and second containers are then swapped as the energy transfer between the gasses reduces sufficiently to reduce the expansion processes, and the method is repeated.

In an additional aspect, the invention affords an apparatus to utilize a cryogenic fluid to operate a prime mover that includes at least one cryogenic liquid reservoir, at least one working fluid liquid condenser, a thermal contact between the cryogenic liquid reservoir and the working fluid liquid condenser, a first heat exchanger pipe, a second heat exchanger pipe, a thermal contact between the first heat exchanger pipe and the second heat exchanger pipe, a first prime mover coupled to the first heat exchanger pipe, a second prime mover coupled to the second heat exchanger pipe; an atmospheric boiler coupled to the second prime mover, at least one working fluid liquid reservoir coupled to the atmospheric boiler, a means for moving working fluid from the at least one working fluid liquid condenser and the at least one working fluid liquid reservoir, and a plurality of valves, situated so as to isolate at least one working fluid liquid reservoir, at least one heat exchanger pipe, at least one working fluid liquid condenser, at least one prime mover, and at least one cryogenic liquid reservoir from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional liquid nitrogen engine;

FIG. 2 is a block diagram of a dual thermodynamic cycle cryogenically powered prime mover system in accordance with the invention, where a cold Rankine cycle engine utilizes the temperature differential between a cryogenic fluid and the ambient temperature;

FIG. 3 illustrates a vapor pressure curve of a working fluid, sulfur hexafluoride (SF₆), that may be used in the invention;

FIG. 4 is a block diagram of the system of FIG. 2 showing a heat exchanger in more detail and where a conventional feed water pump is used;

FIG. 5 is a block diagram of a cryogenic prime mover system, in accordance with an alternative embodiment of the invention where a piston-less feed water pump is used in place of a conventional feed water pump;

FIG. 6A is a diagrammatic view of an implementation of a cryogenic prime mover system, in accordance with the invention;

FIG. 6B is a diagrammatic view showing a first operating state of the system of FIG. 6A;

FIG. 6C is a diagrammatic view of a second operating phase of the cryogenic prime mover system of FIG. 6A;

FIG. 6D is a diagrammatic view of a third operating phase of the cryogenic prime mover system of FIG. 6A;

FIG. 6E is a diagrammatic view of a fourth operating phase of the cryogenic prime mover system of FIG. 6A;

FIG. 6F illustrates a cryogenic prime mover system, in accordance with an alternative embodiment of the invention;

FIG. 7 is a flowchart of a method of operating a cryogenic prime mover system, in accordance with the invention;

FIG. 8 is a flowchart of an alternative method of operating a cryogenic prime mover system in accordance with the invention;

FIG. 9 is a flowchart of a method of operating a cryogenic prime mover system in accordance with a further embodiment of the invention; and

FIG. 10 is a flowchart of a method of operating a cryogenic prime mover system, in accordance with another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention can be constructed from commercially available components. In all of the embodiments disclosed below, different materials could be used for the chambers and reservoirs, including but not exclusively including: various plastics, rubbers, resins, ceramics, and metals. In all of the embodiments disclosed below, different materials could be used for the piping, including but not exclusively including: various plastics, rubbers, resins, ceramics, metals, or other equivalent manmade materials. The heat exchanger could be a low-temperature heat exchanger, a mid-temperature heat exchanger, a high-temperature heat exchanger, or a combination of different types of heat exchangers.

FIG. 1 illustrates a block diagram of a conventional open Rankine cycle liquid nitrogen engine that has been extensively studied at the University of Washington. As shown, the engine has a heat source supplying heat 103 to a heat exchanger 102 that is connected with a pipe 169 to an expander/turbine (e.g., air turbine, generator, or equivalent) 112, which in turn, is connected with an output pipe 167 to an economizer 124. The economizer is connected to an inlet pipe 165 from a liquid nitrogen tank and feed pump 106. The economizer 124 heats up the liquid nitrogen and supplies it by an output pipe 104 to the heat exchanger 102. The engine uses liquid nitrogen stored at a temperature of 77° K. and a pressure of 1 bar. The nitrogen is pumped, as a liquid, up to the system working pressure, and this high pressure liquid nitrogen flows into the economizer 124.

The economizer 124 is a shell-and-tube type heat exchanger where the shell-side fluid is the exhaust from the expander. This has the advantage of providing a frost-free pre-heat to the incoming liquid nitrogen. Once through the economizer 124, the vaporized nitrogen enters the heat exchanger, which has a multi-element, tube-in cross flow configuration. The exterior fluid, i.e., the ambient atmosphere, is drawn through the core of the heat exchanger 102 either by the motion of the engine, e.g., the vehicle in which it is used, or by a fan, depending on the operating regime. This heat exchanger 102 must be able to operate across the normal spectrum of environmental and operating conditions without suffering the adverse effects of frost buildup.

Upon leaving the heat exchanger 102 at 169, the working fluid is a high pressure, near-ambient temperature gas that is injected into the expander/turbine 112, which provides all of the motive work for the system. This expander/turbine can be either a positive displacement engine or a turbine engine. Following expansion, the low pressure nitrogen gas exhaust is warm enough to be used in the economizer 124 to preheat the incoming liquid before finally being vented to the atmosphere.

Heat 103 is supplied to the heat exchanger 102, and nitrogen gas 105 is rejected by the economizer 124. The work output 107 from this engine is from the expander/turbine (e.g., air turbine, generator, or equivalent) 112 and the work input 108 to this engine is used by the tank and feed pump 106. The net work produced by this engine is the difference of the work output 107 and the work input to operate the liquid nitrogen tank and feed pump 106.

The open Rankine cycle operates at critical pressure on the temperature-entropy diagram. Because pressurization of the working fluid occurs in the liquid phase of the fluid, the work required is small in comparison with the available work. One process comprises the pass through the economizer and heat exchanger. Two other processes comprise the isothermal and adiabatic modes of expansion, respectively, that provide the upper and lower limits to the expander's performance. Another process comprises the liquefaction stage, and this occurs remotely at an air processing plant.

In virtually all such known engines, the liquid nitrogen must be stored in a very well insulated tank having a reservoir capacity of 50 to 100 liters of liquid nitrogen, and ambient heat is used to convert the liquid nitrogen to an expanding gaseous nitrogen to run an air turbine which emits gaseous nitrogen as a benign exhaust. However, the miles per liter of liquid nitrogen attainable in a vehicle powered by this engine is typically been on the order of less than one mile per liter of liquid nitrogen used, thereby requiring impractically large and cumbersome tanks of liquid nitrogen. Accordingly, this technology has limited practical application, and has mainly been only a technological curiosity.

As will be described below, the invention provides a system which has particular utility for more efficiently operating energy conversion apparatus, such as a prime mover for generating useful mechanical power, or to do work such as generate chemical energy, compress gases, or drive generators to produce either AC or DC electricity. Other embodiments of the invention provide apparatus or systems that are useful for supplying cryogenic fluids, i.e., fluids generated from cryogenic liquefaction of substances such as nitrogen or oxygen, liquefied natural gas, liquefied hydrocarbon gases, any mixture of the above, or equivalents.

More particularly, the invention may employ a modified closed Rankine cycle engine coupled to a cryogenic fluid in an open cycle Rankine cycle process to afford a dual thermodynamic cycle system. The heat needed to boil the cryogenic fluid is supplied by driving the phase transition of a working fluid, such as SF₆, CO₂, and others, as will be discussed later, from the gas phase to the liquid phase. A reservoir of the working fluid is raised to the ambient temperature (or higher if an external heat source is utilized). This hot high pressure gas is run through a prime mover (gas turbine, expander, or equivalent) where the pressure is lowered and then feed into the condenser where the gas to liquid phase transition is driven by giving off heat to boil off the cryogenic fluid.

A cold engine differs from a conventional hot Rankine (or steam) engine. In the hot engine, the cold side of the engine is close to infinite (the heat is dumped into the ambient air temperature) and the “hot” side of the engine is limited by the available fuel and the temperature at which that fuel burns. In the case of the “cold” engine, this is reversed. There is a finite supply of fuel on the cold side of the engine, and an infinite amount of heat on the hot side is available from the ambient environment. The invention utilizes this difference in a novel system and method to achieve considerable improvements in efficiency over convention approaches by warming a working fluid when it is moved from the liquid cold side of the engine to the liquid hot side of the engine.

As will be described, the invention also affords a novel piston-less pump for moving working fluid between hot and cold sides of the system, without having to rely on a conventional feed water pump as used in the Rankine cycle engine. The piston-less feed fluid pump has several advantages over the conventional feed water pump in a cold engine. It uses the fact that there is an infinite supply of heat for the operation of the engine. Cold working fluid can be directly exposed to the hot side of the engine and brought to working temperature simply by exposing the cold working fluid to ambient in a heat exchanger, similar to the method used in the prior art liquid nitrogen powered vehicles. Hot working fluid gas moves through the prime mover generating power, and is condensed back to a liquid by boiling the cryogenic fluid. The gaseous cryogenic fluid is at a high pressure due to the heat supplied by the phase transformation of the working fluid from gas to liquid. This high pressure gas is still very cold and can be used in a heat exchanger to cool the low pressure gaseous working fluid that is exiting a first prime mover and moving to the low pressure in a condenser. This will result in warm high pressure gaseous cryogenic fluid being available for another thermodynamic cycle system and to work in an expander similar to an open Rankine cycle. The low pressure gaseous cryogenic fluid is expelled into the atmosphere from the output of a second prime mover.

FIG. 2 illustrates a block diagram of a dual thermodynamic cycle cryogenically fueled prime mover system in accordance with a first embodiment of the invention. As will be explained, the invention may comprise a first system or system part operating according to a first thermodynamic cycle, such as an open Rankine cycle, coupled to a second system or system part operating on a second thermodynamic cycle, such as a closed Rankine cycle.

As shown in FIG. 2, the overall system may comprise a cryogenic liquid reservoir 202 in thermal contact, i.e., thermal communication, 204 to a working fluid liquid condenser 206, a working fluid pump 208, a heat exchanger 209 comprising first and second half heat exchangers 210 and 212 in thermal contact, i.e., thermal communication, 211. Half heat exchanger 210 may be coupled to the cryogenic liquid reservoir 202; and half heat exchanger 212 may be coupled to the working fluid liquid condenser 206. Condenser 206 may be coupled to the working fluid pump 208, and the pump may be coupled to a working fluid liquid reservoir 214. The reservoir 214 may be coupled to an atmospheric boiler 228 which is coupled to a prime mover 222, and that, in turn, is coupled to the half heat exchanger 212. The atmospheric boiler 228 coupling to prime mover 222 may be through a high pressure gas line 224, and the atmospheric boiler may further be coupled via a pressure equalization line 230 to the working fluid liquid reservoir 214. In a preferred embodiment, the prime mover 216 may be a radial piston air motor, such as is available from Cooper Power Tools, for example, or another type of motor or generator. Prime mover 216 which is coupled to the half heat exchanger 210 may produce output power 218, and prime mover 222 may produce output power 226. Prime mover 216 may also have an exhaust 220 for exhausting the gas phase of the cryogenic fluid to the atmosphere.

The open cycle system part of the overall system shown in FIG. 2 includes the cryogenic liquid reservoir 202, the half heat exchanger 210 and the prime mover 216. The closed system part comprises the working fluid liquid condenser 206, the working fluid pump 208, the working fluid reservoir 214, the atmospheric boiler 228, the prime mover 222 and the half heat exchanger 212. The coupling of the two system parts comprises the thermal contact 204 between reservoir 202 and condenser 206, and the thermal contact 211 between the two half heat exchangers 210 and 212.

As will be explained more fully below, the invention employs the heat exchange between a cryogenic fluid and a working fluid to cause phase transitions in the fluids between liquid and gaseous states, and employs the high pressure gases resulting from such transitions to drive prime movers to produce work or otherwise convert the thermal energy to another form. The invention optimizes this operation by coupling the two different thermodynamic systems together in a way that substantially improves efficiency by capturing and using thermal energy that would otherwise be lost in the two systems. This requires thermal communication, and preferably physical contact, between containers of the fluids in the two systems, as by thermal contacts 204 and 211, and as will be described more fully below.

The thermal contact 204 between the cryogenic liquid reservoir 202 and the working fluid liquid condenser 206 may be achieved by placing the cryogenic liquid reservoir 202 in heat conductive proximity to (preferably contact with) the working fluid liquid condenser 206 to enable thermal communication and good exchange of heat between the reservoir and condenser. The thermal contact 204 may comprise heat pipes or equivalent heat transporters to provide thermal communication between the cryogenic liquid reservoir 202 and the working fluid liquid condenser 206. Similarly, the thermal contact 211 between the half heat exchanger 210 and the half heat exchanger 212 may be achieved by placing the half heat exchangers in contact with one another.

In the embodiment of the invention shown in FIGS. 2 and 4, the working fluid pump 208 may be a conventional piston pump, for example, and assists in transporting the working fluid liquid from the condenser 206 to the reservoir 214. As will be described later, the invention affords, and other embodiments preferably employ, a piston-less pumping arrangement instead of a conventional piston pump for moving the working fluids.

Suitable fluids and their utility in the invention are explained in more detail below. In some embodiments of the invention, one fluid may be used as a working fluid, and a second fluid may be used as a reservoir fluid. Several possible fluids that may be utilized for the working fluid, include but are not limited to sulfur hexafluoride (SF₆), carbon dioxide (CO₂), liquefied natural gas (LNG), a mixture of the above, or other non-aqueous fluids that can be liquefied at low pressure at a temperature above the temperature of liquid nitrogen and at high pressure and standard temperature, but also exists as a gas at standard temperature and low pressure. The cryogenic fluid may comprise liquid nitrogen, for example, which is used to condense a working fluid, such as SF₆ (or another working fluid, as described below). The energy needed to drive the gas-to-liquid phase transformation in the SF₆ working fluid may be supplied by an equivalent liquid-to-gas phase transition for the liquid nitrogen. The liquid nitrogen may be maintained well below the critical point, but still pressurized to several hundred pounds per square inch (psi). There may be a heat exchange between the cold nitrogen gas (cryogenic fluid) and the warm SF₆ gas (working fluid) to improve overall system efficiency, as will be explained.

The prime mover 216 being fueled by the cryogenic fluid may exhaust the gas phase of the cryogenic fluid to the atmosphere at 220. Alternatively, the prime mover 216 may transmit the gas phase of the cryogenic fluid back into the system, as to a storage reservoir, for reuse or recycling. Prime mover 216 produces output power at 218 from the gas expansion of the cryogenic fluid; and prime mover 222 produces output power at 226 from the gaseous expansion of working fluid, thereby affording a system that operates on two thermodynamic cycles. This output power can be converted to mechanical power, for use in a vehicle such as an automobile, for example, to chemical energy, or used to compress gases, and/or to generate electricity. The working fluid in the engine can be any fluid having a suitable vapor pressure curve, as shown in FIG. 3. Sulfur hexafluoride is one example of a suitable working fluid.

FIG. 3 illustrates a vapor pressure (PV) curve for sulfur hexafluoride (SF₆) which may be used as a working fluid in the invention. The vertical axis 302 in FIG. 3 indicates the vapor pressure of the working fluid SF₆, and horizontal axes indicate the temperature. The curve 304 indicates the relation between the vapor pressure of the working fluid SF₆ and the temperature. Horizontal axis 306 indicates the temperature in degrees Kelvin (K), and horizontal axis 308 indicates the temperature in degrees Celsius (C). The working fluid is preferably a fluid chosen to have a high pressure at the working temperature (typically ambient) and a much lower pressure at the cryogenic temperature in the condenser. As mentioned above, both CO₂ and SF₆ adhere well to these criteria.

As is well known, the process of evaporation in a closed chamber will proceed until there are as many molecules returning to the liquid as there are escaping. At this point the vapor is said to be saturated, and the pressure of that vapor (usually expressed in mmHg) is referred to as the saturated vapor pressure. Since the molecular kinetic energy is greater at higher temperatures, more molecules can escape the surface and the saturated vapor pressure is correspondingly higher. If instead the chamber and the liquid are open to the air, then the vapor pressure is a partial pressure along with the other constituents of the air. The temperature at which the vapor pressure is equal to the atmospheric pressure is called the boiling point. The phase change of a fluid from a liquid to a vapor (and the reverse process) can be utilized in multiple ways by the invention, as will be described below.

Sulfur hexafluoride (SF₆) is a preferred working fluid because of its pressure at ambient (30-50 bar) and its ability to be pumped by cryogenic liquid nitrogen in sulfur hexafluoride (SF₆) recovery systems. The pressure vapor (PV) curve for carbon dioxide (CO₂) is similar to that of SF₆ and CO₂ is a good alternative to SF₆. Additionally, the invention has the advantage of capturing CO₂ in the engine and removing it from the atmosphere. This is particularly advantageous if the engine finds substantial use in many applications and vehicles. Depending on the application and the conditions in which the invention is used, standard PV curves as shown in FIG. 3 may be employed to identify other appropriate working fluids.

FIG. 4 is a block diagram of the system of FIG. 2 showing more details of the heat exchanger 209 and its relation to other components of the system, and will be used for describing the operation of the system.

As described in connection with FIG. 2, heat exchanger 209 may comprise two half heat exchangers 210 and 212 in thermal contact with one another. As indicated in FIG. 4, the heat exchanger 209 may comprise an enclosure housing the two half heat exchangers 210 and 212 in thermal contact with each other by a common thermally conducting wall 211 separating the two half heat exchangers. As mention above, the two heat exchangers 210 and 212 may comprise heat pipes of heat conductive material such as copper, or some other metal or alloy having good heat transfer characteristics, preferably joined together by the common wall 211 in such a fashion as to maximize the heat transfer from one pipe to the other pipe. The heat transfer is a function of the area of the thermal connection between the two pipes.

As shown in the figure, half heat exchanger 210 may have a high pressure cold end coupled to the cryogenic liquid reservoir 202 for receiving cold cryogenic gas from the reservoir 202. This cold cryogenic gas is produced by adding heat to the cryogenic liquid from the working fluid in condenser 206 via thermal contact 204 to expand the cryogenic liquid to a gas. Half heat exchanger also has a high pressure hot end coupled to the prime mover 216 for providing high pressure hot cryogenic gas to the prime mover. The cryogenic gas flowing through half heat exchanger 210 receives additional heat via thermal contact 211 from the hot working fluid gas flowing in half heat exchanger 212 to increase its internal thermal energy. This additional thermal energy substantially increases efficiency of the system by increasing the energy in the hot gas flowing to prime mover 216.

Half heat exchanger 212 has a low pressure hot end coupled to the prime mover 222 for receiving the hot exhaust working fluid gas from the prime mover, which is exhausted at low pressure from the prime mover. As the hot exhaust gas passes through the half heat exchanger 212, it is chilled by the transfer of heat to the expanded cryogenic fluid gas in half heat exchanger 210, and exits the half heat exchanger 212 at a low pressure cold end coupled to the working fluid liquid condenser 206. The chilled working fluid gas enters the condenser 206 where it is further chilled and condensed to a liquid by the heat transfer via thermal contact 204 to the cryogenic liquid in reservoir 202, which causes the cryogenic liquid to undergo a liquid-to-gas phase transition, as described above.

Advantageously, in accordance with the invention, the hot working fluid gas from prime mover 222 that enters and flows through half heat exchanger 212 heats the cryogenic gas that flows through half heat exchanger 210 to prime mover 216 to further increase the internal energy of the cryogenic gas, as explained. The cryogenic gas exiting the high pressure hot end of the heat exchanger 210 to prime mover 216 will be heated to a temperature closer to the temperature of the working fluid entering heat exchanger 212 from prime mover 222, thereby providing more to the prime mover 216 and substantially increasing its output power. In fact, the increased temperature differential over ambient resulting from heating of the cryogenic gas by hot working fluid gas in the heat exchanger 209 results in an increase in work output of the order of three to four times over conventional open Rankine cycle systems. Moreover, by the time the working fluid gas from prime mover 222 exits the cold end of half heat exchanger 212, it will be much nearer in temperature to the cryogenic fluid than when it entered the heat exchanger due to the heat transfer to the cryogenic liquid flowing through half heat exchanger 210, thereby facilitating the gas-to-liquid phase transition in the condenser. This heat exchange between the two fluids improves system efficiency.

The working fluid may be pumped by a conventional working fluid pump from the condenser to the working fluid liquid reservoir, where the temperature of the working fluid liquid will be near but somewhat less than ambient and its pressure will be of the order of 40-50 bar, when the working fluid is sulfur hexafluoride. From the reservoir 214, the working fluid flows to the atmospheric boiler 228 where it receives thermal energy and undergoes a phase change from a liquid to a vapor and forms the high pressure hot working fluid gas that drives the prime mover 222. As noted, this high pressure hot gas is exhausted from the prime mover to the heat exchanger 212. The atmospheric boiler 228 may be coupled back to the reservoir 214 by a pressure equalization line 230, as shown.

FIG. 5 illustrates a block diagram of an alternative embodiment of a cryogenic prime mover system in accordance with of the invention. As shown in FIG. 5, this alternative embodiment may be similar to the system of FIGS. 2 and 4, comprising a cryogenic liquid reservoir 202, a thermal contact 204 to a working fluid liquid condenser 206, a working fluid pump 208, a heat exchanger 210 coupled to the cryogenic liquid reservoir 202, and a heat exchanger 212 coupled to the working fluid liquid condenser 206. However, the embodiment of FIG. 5 differs in several significant respects from those embodiments.

In addition to a first working fluid liquid reservoir 214, the system of FIG. 5 may also comprise a second working fluid liquid reservoir 215 coupled (as at 217) to the first working fluid reservoir 214, and the two reservoirs may be coupled to the condenser 206. Additionally, the working fluid pump 208 of FIG. 2 and 4 which was used to circulate working fluid may be eliminated. Instead, as will be described below in connection with FIGS. 6A-E, the reservoirs 214 and 215 and their interconnections in the system of FIG. 5 afford a piston-less pumping operation that moves the working fluid in the system without the necessity of a conventional pump. Additionally, the system of FIG. 5 may comprise a pump 604 and a hot storage collector 602 coupled together with the atmospheric boiler 228, as shown. The pump 604 and hot storage collector 602 are advantageous in allowing additional heat to be transferred to the atmospheric boiler to raise its temperature above ambient, by pumping a transfer liquid, such as oil or liquid sodium, from the boiler through the hot storage collector 602. Hot storage collector 602 may comprise, for instance, cooling fins as in a radiator that operates at ambient temperature. This allows heat to be captured and added to the boiler to improve its efficiency and to increase the thermal energy in the working gas flowing to prime mover 222.

As also shown in the figure, the first and second working fluid liquid reservoirs 214 and 215 may both be coupled to atmospheric boiler 228. The atmospheric boiler may couple back to the first and second working fluid liquid reservoirs 214 and 215 via a pressure equalization line 230, and the boiler may supply high pressure hot gaseous working fluid to prime mover 222 via high pressure gas line 224. The two working fluid liquid reservoirs may also be connected together and to the boiler by a line 217.

As explained above, the working fluid from the reservoirs 214, 215 undergoes a phase change from a liquid to a vapor upon the application of thermal energy in atmospheric boiler 228. This vapor is at high pressure and may be used to drive the prime mover 222 to produce work 226, as explained above, and the exhaust from the prime mover may be supplied to the half heat exchanger 212.

FIGS. 6A-6E are more detailed diagrammatic views of a dual thermodynamic cycle cryogenically fueled prime mover system in accordance with the invention as shown in FIG. 5, and illustrate various phases (states) of the system and its control as the system progresses through an operating cycle. The figures also illustrate the piston-less pumping aspect of the invention.

As described, the working fluid portion of the system may comprise a closed loop system in which the working fluid is recycled and reused as the system, whereas the cryogenic portion of the system is an open system that does not reuse cryogenic fluid, but exhausts it to the atmosphere from a prime mover (motor/generator). These two systems operate according to different thermodynamic cycles that may comprise, respectively a closed Rankine cycle and an open Rankine cycle. The arrangement of the system components and the control by the valves of the pressure and flow through the working fluid reservoirs (tanks 1 and 2) affords a piston-less pump arrangement that circulates fluid through the closed cycle working fluid portion of the system without the necessity of a conventional pump.

Referring to FIG. 6A, cryogenic liquid reservoir 202 may be a container of liquid nitrogen (LN₂), for example, in thermal communication with the working fluid liquid condenser 206 for SF₆ or CO₂, for example, SF₆ being shown in the figure. The thermal communication may be effected by the thermal contact 204 between the cryogenic liquid reservoir 202 and the working fluid liquid condenser 206. The first and second working fluid liquid reservoirs 214 and 215 may comprise two tanks connected to the condenser 206 by valves V1 and V2, and connected together by a valve V3. The atmospheric boiler 228 may comprise an evaporator which receives heat 606, as from the atmosphere or another heat source to convert the working liquid to a gas, and the two reservoir tanks may also be connected to the evaporator by a line 232 having control valves V4 and V5, and by pressure equalization lines 234 and 236 (corresponding to line 230 of FIGS. 4-5) respectively having control valves V6 and V7.

A prime mover (e.g., motor/generator) 216 may be coupled to the cryogenic liquid reservoir 202 through an evaporator 203, and a prime mover (e.g., motor/generator) 222 may be coupled to the working fluid liquid condenser 206 and to the atmospheric boiler (evaporator) 228 via a line 224 and control valve V8. The prime mover 216 has an exit to atmosphere 220 for releasing gas from the cryogenic fluid produced by evaporator 203.

Valves V1 and V2 control the flow of working fluid into working fluid liquid reservoirs (tanks) 214 and 215, respectively. Valve V3 controls the pressure equalization and the flow of working fluid between working fluid liquid reservoirs 214 and 215. Valves V4 and V5 respectively control the flow of working fluid from working fluid liquid reservoirs 214 and 215 into the evaporator (atmospheric boiler) 228. Valves V6 and V7 may control the equalization of pressure in the working fluid liquid reservoirs 214 and 215, respectively, relative to the evaporator (atmospheric boiler). Valve V8 controls the flow of working fluid gas between the motor/generator (prime mover) 222.

FIG. 6B is a simplified view of the cryogenic prime mover system of FIG. 6A showing the system in a first operating phase (state). FIG. 6B shows the system with all the valves in the closed position. Reservoir 215 (i.e., tank 2) is substantially full of cold working fluid liquid, and reservoir 214 (i.e., tank 1) is more or less empty of working liquid but contains high pressure warm working gas. Valves V1 and V2 are shut-off to stop the flow of working fluid into the working fluid liquid reservoirs 214 and 215, respectively. Valve V3 is shut-off to stop the flow of working fluid between the working fluid liquid reservoirs and to maintain the separate pressures in each reservoir. Valves V4 and V5 are shut-off to stop the flow of working fluid from the working fluid liquid reservoirs into the evaporator (atmospheric boiler) 228, and valves V6 and V7 are shut-off to stop the equalization of pressure in the working fluid liquid reservoirs relative to the evaporator (atmospheric boiler) 228. Valve V8 is shut-off to stop the flow of working fluid gas to the motor/generator (prime mover) 222. The system is shown in a quiescent state in FIG. 6B.

FIG. 6C illustrates a next second phase of the system from the state shown in FIG. 6B in which valves V1, V5, V7 and V8 are opened. Working fluid liquid reservoir 215 (tank 2) is substantially full of cold working fluid. The working fluid from tank 2 may feed the evaporator 228, which expands the working fluid to supply hot gas to the generator 222. The hot gas from the generator 222 is condensed and liquefied in the condenser 206 (which is in thermal contact 204 with the cryogenic liquid reservoir 202, as shown in FIG. 6A), and stored as cold working fluid liquid in reservoir 214 (tank 1). Not shown in the figure, there is preferably a heat exchanger in the line between tanks 1 and 2 and the evaporator 228 that uses the hot gas exhausted from the generator 222 to the condenser to heat the warm working fluid flowing to the evaporator. This increases the energy in the working fluid to the evaporator, and removes heat from the exhaust gas to the condenser to increase system efficiency.

Valve V1 may be open to allow working fluid from condenser 206 to flow into working fluid reservoir 214, and V2 may be shut-off to stop the flow of working fluid into working fluid reservoir 215. Valve V3 is shut-off to stop the flow of working fluid between working fluid reservoirs 214 and 215 and to maintain the reservoir pressures. Valve V4 is shut-off, and V5 is open to permit the flow of working fluid from working fluid reservoir 215 into evaporator 228. Valve V6 is shut-off and V7 is open to permit the equalization of pressure in the working fluid liquid reservoir 215 relative to the evaporator 228. Finally, valve V8 is open to permit the flow of hot working fluid gas from the evaporator to the motor/generator 222.

FIG. 6D illustrates a next third phase of the system of FIG. 6A, in which working fluid reservoir 214 is substantially full of cold working liquid, and working fluid reservoir 215 is substantially empty of liquid but contains hot gas which filled the reservoir from reservoir 214 when valve V3 was opened. With valve V3 open, the pressure is equalized between reservoirs 214 and 215. When the pressure is equalized, valve V3 may be closed and the system will be ready to reverse the cycle. Valves V1 and V2 may be shut-off to stop the flow of working fluid into working fluid liquid reservoirs 214 and 215, respectively. Valves V4 and V5 may be shut-off to control the flow of working fluid from working fluid liquid reservoirs 214 and 215 into the evaporator 228. Valves V6 and V7 are shut-off to stop equalization of the pressure in the working fluid liquid reservoirs 214 and 215 relative to the evaporator 228. Valve V8 is shut-off to stop the flow of working fluid gas to the motor/generator (prime mover) 222.

FIG. 6E illustrates a fourth phase of the system that is similar to the state shown in FIG. 6A, where the system flow is reversed from the state of FIG. 6D. Working fluid reservoir 214 (tank 1) is substantially full of warm working liquid (warmed by the hot gas flow from tank 2 in the phase shown in FIG. 6D) and may be used to feed the evaporator 228 and power the generator 222. Working fluid reservoir 215 (tank 2) is substantially empty of liquid, but contains cold working fluid from the condenser 206. Valve V1 is shut-off and V2 is open to control, respectively, the flows of working fluid into working fluid reservoirs 214 and 215. Valve V3 is shut-off to stop the flow of working fluid between working fluid reservoirs 214 and 215. Valve V4 is open and V5 is shut-off to control the flow of working fluid from working fluid liquid reservoirs 214 and 215 into evaporator 228. Valve V6 is open for the equalization of pressure in the working fluid reservoir 214 relative to evaporator 228 and to enable the working fluid to flow from reservoir 214 to the evaporator, and valve V7 is closed. Valve V8 is open to permit the flow of working fluid gas to the motor/generator 222.

The working liquid in working fluid reservoir 214 (tank 1) is warm because of the flow of hot gas from reservoir 215 and because the liquid is heated to the ambient atmospheric temperature in the tank. As described above, there is preferably a heat exchanger in the line between the working fluid reservoir and the evaporator that uses the hot exhaust gas from the generator 222 to preheat the working fluid flowing to the evaporator. Moreover, in an alternative embodiment (not illustrated) the cold fluid from working fluid reservoirs may be used in either condenser 206 assist in condensing the gas from the generator, or may be used in another system having tanks 3 and 4 arranged similar to that shown and run another cycle and extract additional energy from the working fluid.

As will be appreciated from the foregoing description of the operating states of the system as shown in FIGS. 6A-E, the movement of working fluid between the reservoir tanks 1 and 2 (214 and 215) and its circulation through the system, is controlled by the pressure and temperature differentials between the two working fluid reservoirs and between other system components, and by the positions of the various valves in the interconnecting lines. By appropriate control of the valves, the system can be cycled through its various states and the working fluid can be circulated through the system by using these pressure differentials without the necessity of a conventional pump. The arrangement of the system components and valves effectively affords a piston-less (non-moving) pump that effects transport of fluids based on the pressure and temperature differentials, thereby avoiding the necessity of a conventional pump, such as a feed water pump used in conventional Rankine cycle engines. As will be appreciated from the foregoing, this piston-less pump has more general applicability to other types of systems employing fluids that expand with heat and condense with cold to produce varying pressures differentials as the system progress through an operating cycle.

FIG. 6F illustrates an open loop cryogenic prime mover system in accordance with an alternative embodiment of the invention. FIG. 6F illustrates a simpler embodiment of that shown in FIG. 6A-6E. This system uses a single container or tank of hot working fluid 214 which is raised to its boiling point for a given input pressure to a motor/generator 222 (prime mover) by a heating tape 608 surrounding the container. The working fluid is expanded in the motor/generator, and then re-condensed in a second container or tank 215 disposed in an insulated dewar 614, pressure sealed at 612, that contains a cryogenic fluid such as liquid nitrogen, LN₂. As the liquid nitrogen absorbs heat from the working fluid, it vaporizes into a hot gas, and pressurizes the interior of the sealed dewar. The pressurized liquid nitrogen may be supplied through a pressure regulator 616 to another motor/generator (prime mover) 216, and exhausted at 220 to the atmosphere. Power is generated both by the movement of the working fluid from tank 214 through motor/generator 222 to tank 215, as well as by the movement of warm gaseous cryogenic fluid through prime mover 216 as it is vented at 220 to the atmosphere.

At the start of the cycle, the tank 214 may be substantially full of working liquid (i.e., working fluid), such as the previously mentioned fluids SF₆ or CO₂. This working liquid may be transitioned (changed) to the gas phase using heat added by the heating tape 608 to create a high-pressure working gas. The working gas flows through the generator 222 to the low-pressure side of the generator and into container 215, where it is re-liquefied in the container. The phase transition from gas-to-liquid is exothermic, and the energy to drive this reaction is provided by the liquid-to-gas transformation occurring in the LN₂ pressure vessel 202 comprising the insulated dewar 614.

As the cycle proceeds, work is done in the two generators 222 and 216 as the high-pressure working fluid in the tank 214 transitions to the gas phase and moves through the motor/generator and into the tank 215. Work is also done by the pressurized gas created in the LN₂ pressure vessel 202 as it is vented through the motor/generator 216 to the atmosphere.

When the cycle is completed, substantially all of the liquid working fluid may have moved from working fluid tank 214 into working fluid tank 215. The tanks 214 and 215 may then be disconnected from the system and physically exchanged or switched places so that the original right-most tank (in the figure) 215 becomes the new left-most heated tank 214, once again full of the working fluid, and the original left-most tank 214 becomes the new right-most tank 215 ready to be chilled to condense the working gas to a liquid state.

As will be appreciated, the embodiment of FIG. 6F is a manual system, rather than an automatic system such as FIGS. 6A-E, since following one cycle of operation, the system must be “reset” before a second cycle may be commenced. Moreover, while the working fluid portion of the system is a closed system, since working fluid may be recycled and reused in a subsequent cycle, the cryogenic portion of the system is an open system since liquid nitrogen is not recycled, but rather is exhausted to the atmosphere. Therefore, the liquid nitrogen dewar will have to be recharged periodically.

The invention can be used to generate electrical energy, either for fixed locations, e.g., for residential or business use, or for moving vehicles, e.g., for automobiles, trucks, etc. Generating electrical energy in large quantities for an electrical power grid using conventional power generation approaches is not trivial, especially for AC electricity power grids, since large amounts of AC electrical energy needed by an electricity power grid cannot be readily stored. Therefore the energy taken from an electrical power supply grid must be equal to the energy being delivered.

Cryogenic fluid reservoir systems in accordance with the invention solve this problem by storing electrical energy as potential energy in cryogenic fluids. Systems of the invention may generate cryogenic fluids at times of surplus energy on an electrical supply grid, typically at night, store the energy as potential energy, and then release the potential energy through an electrical generator at times of high demand. Use of electric generator turbines allows direct energy conversion to AC electrical power. Electrical DC to AC conversion as used in many conventional alternative energy systems is not required, thereby significantly reducing the complexity, reliability problems, and cost of construction and maintenance of energy generation plants, as compared to conventional DC electricity supply systems.

FIG. 7 is a flowchart of a method of converting thermal energy to another useful form of energy, in accordance with the invention. The method begins at 702. At 704, thermal energy is transferred into a cryogenic fluid from a working fluid to vaporize a portion of the cryogenic fluid and generate a first working fluid gas. At 706, the first gas may be used to operate a first prime mover to convert the energy of the first gas to a different form. At 708, at least a portion of the working fluid may be vaporized to create a second working fluid gas. At 710, the second gas may be utilized to operate a second prime mover. At 712, the second gas may be routed through a heat exchanger to substantially condense the second gas back into working fluid liquid. The method ends at operation 714.

FIG. 8 illustrates a flowchart of an alternative method of converting thermal energy to another form of energy that is substantially similar to the method of FIG. 7. Steps 802-806 and 812-817 of FIG. 8 are substantially the same as steps 702-714 of FIG. 7. The difference in the methods of FIGS. 7 and 8 is that the method of FIG. 8 includes, following the step at 806 (where the first gas is used to operate a prime mover), the additional step at 808 of moving a portion of the working fluid to a reservoir. The remainder of the steps of the method may be the same as shown in FIG. 7.

FIG. 9 illustrates a flowchart of another method of generating energy in accordance with the invention. The method begins at 902. At 904, a first portion of a working fluid is stored in a first working fluid reservoir. At 906, a second portion of the working fluid is stored in a second working fluid reservoir. At 908, thermal energy is transferred into a cryogenic fluid, for instance from the first portion of working fluid, to vaporize a portion of the cryogenic fluid to generate a first gas. At 910, the first gas may be used to operate a first prime mover. Next, at 912, the first portion of the working fluid may be separated from the second portion of the working fluid, and at 914 the first portion of the working fluid may be vaporized to generate a second gas. At 916, the second gas may be utilized to operate a second prime mover. This prime mover can be used to convert the energy in the second gas to another form of energy, as, for example, to generate mechanical power, electrical power, compress gases, or create chemical energy. At 918, the second gas may be routed through a heat exchanger to substantially condense the second gas into the first portion of the working fluid. Next, at 920, the preceding three operations at 914, 916 and 918 may be repeated using the second portion of the working fluid rather than the first portion, after swapping the roles of the first and second working fluid reservoirs. The method ends at 922.

FIG. 10 is a flowchart of a method of generating electricity in accordance with the invention. The method begins at 1002, and at 1004, thermal energy is transferred into a cryogenic fluid from a working fluid to vaporize a portion of the cryogenic fluid and generate a first gas. At 1006, the first gas may be utilized to operate a first prime mover that may include a first generator. A 1008, at least a portion of the working fluid may be vaporized to create a second gas, which may be included, as indicated at 1010, to operate a second prime mover that may include a second generator. At 1012, the second gas may be routed through a heat exchanger to substantially condense the second gas into the working fluid. The method ends at 1014.

As will be appreciated from the foregoing, while the invention has been described with reference to preferred embodiments, various changes in these embodiments may be made without departing from the spirit and principles of the invention, the scope of which is defined in the appended claims. 

1. A method of converting thermal energy comprising: transferring thermal energy into a cryogenic fluid from a first portion of working fluid to expand the cryogenic fluid and create a first gas and to convert the first portion of working fluid to a working liquid; transferring thermal energy into a second portion of the working fluid to expand the second portion of the working fluid and create a second gas; utilizing the second gas to operate an energy conversion apparatus; transferring additional thermal energy to the first gas from exhausted second gas from the energy conversion apparatus to increase the energy in the first gas; and utilizing the first gas with said increased energy to operate another energy conversion apparatus.
 2. The method of claim 1, wherein said transferring of additional thermal energy to the first gas comprises substantially reducing the temperature of said exhausted second gas by a heat exchange between said first and exhausted second gasses.
 3. The method of claim 2, wherein said transferring of additional thermal energy comprises flowing said first gas and said exhausted second gas through a common heat exchanger that provides thermal communication between said gasses.
 4. The method of claim 1, wherein said cryogenic fluid comprises cryogenic fluid from a cryogenic liquid reservoir, said first portion of working fluid comprises said exhausted second gas, and wherein said transferring thermal energy into said cryogenic fluid comprises extracting said thermal energy from the exhausted second gas to create said first gas.
 5. The method of claim 4, wherein said extracting thermal energy comprises transferring heat from said exhausted second gas to condense said exhausted second gas to said working liquid.
 6. The method of claim 1, wherein said transferring thermal energy into a cryogenic fluid comprises transferring heat from said first portion of working fluid into a cryogenic liquid in a reservoir.
 7. The method of claim 1, wherein said transferring thermal energy into said second portion of working fluid comprises expanding said second portion of working fluid in an atmospheric boiler.
 8. The method of claim 7 further comprising adding heat to said atmospheric boiler from a heat collector to increase the temperature of said atmospheric boiler above ambient.
 9. The method of claim 1, wherein said portions of working fluid comprise working fluid in different parts of a closed system that operates on a first thermodynamic cycle.
 10. The method of claim 9 further comprising using pressure and temperature differentials in said closed system to circulate said working fluid in said closed system without using a pump.
 11. The method of claim 10 further comprising moving said circulating working fluid between first and second containers by exchanging pressures in said containers and by controlling inlet and outlets of said containers.
 12. The method of claim 9, wherein said cryogenic fluid and said first gas comprise different phases of the cryogenic fluid in an open system operating on a second thermodynamic cycle.
 13. The method of claim 1, wherein one or both of said first and second energy conversion apparatus comprises a prime mover.
 14. The method of claim 1, wherein the working fluid is selected from the group consisting of sulfur hexafluoride, carbon dioxide, liquefied natural gas, and a mixture of the above.
 15. A method of converting thermal energy comprising: expanding a portion of a working fluid in a first system operating according to a first thermodynamic cycle to create a working gas for operating first energy conversion apparatus; expanding a portion of a cryogenic fluid in a second system operating according to a second thermodynamic cycle to create another gas for operating second energy conversion apparatus, said second system being coupled to said first system and said expanding comprising transferring heat from the working gas to said portion of cryogenic fluid to create said other gas; and transferring additional heat from said working gas in said first system to said other gas in said second system to increase the internal energy of said other gas.
 16. The method of claim 15, wherein said transferring heat from said working gas to expand said portion of cryogenic fluid comprises substantially condensing said working gas.
 17. The method of claim 15, wherein said first-mentioned expanding to create said working gas comprises reducing the pressure and increasing the temperature of said working fluid in an atmospheric boiler.
 18. The method of claim 15, wherein said transferring additional heat to said other gas comprises passing said working gas and said other gas through a common heat exchanger.
 19. A method of converting thermal energy, comprising: transferring heat into a working fluid in a first container to expand a portion of the working fluid to create a first gas; operating a first prime mover using said first gas; exhausting said first gas from said first prime mover into a second container immersed in a cryogenic fluid in a third container; transferring heat from the exhausted first gas to said cryogenic fluid to expand a portion of said cryogenic fluid to create a second gas; operating a second prime mover utilizing the second gas; and swapping said first and second containers and repeating said foregoing steps.
 20. A system for converting thermal energy, comprising: a cryogenic fluid reservoir; a condenser for a working fluid, the condenser and the cryogenic fluid reservoir being in thermal communication for the transfer of heat to cryogenic fluid in said cryogenic fluid reservoir to expand a portion of the cryogenic fluid to a first gas; a boiler for transferring heat to working fluid from said condenser to expand the working fluid to a second gas, the second gas operating energy conversion apparatus, and the energy conversion apparatus exhausting said second gas; a heat exchanger receiving the first gas and the exhausted second gas for transferring thermal energy from said exhausted second gas to the first gas to increase the energy in the first gas; and the first gas with increased energy operating another energy conversion apparatus.
 21. The system of claim 20, wherein said condenser receives exhausted second gas from the heat exchanger, said exhausted second gas comprising said working fluid, and wherein said heat transfer to the cryogenic fluid condenses said working fluid to a working liquid, said working liquid comprising said working fluid in said boiler.
 22. The system of claim 21 further comprising first and second working fluid reservoirs being connected together and to said condenser and to said boiler by a plurality of lines containing control valves to enable the control of working fluid through said lines.
 23. The system of claim 20, wherein said heat exchanger comprises heat conductive pipes through which said gasses pass, the pipes being in thermal communication for the exchange of thermal energy.
 24. The system of claim 23, wherein said cryogenic reservoir, said heat exchanger and said other energy conversion apparatus comprise a first system part that operates according to an open thermodynamic cycle, and said condenser, said boiler, said heat exchanger and said first mentioned energy conversion apparatus comprise a second system part the operates according to a closed thermodynamic cycle, said system parts being coupled for the exchange of thermal energy.
 25. A method of operating a system that operates on a closed thermodynamic cycle to circulate fluids through the system without using a pump, the fluids comprising fluids that expand to a gas and condense to a liquid upon the transfer and removal of heat, the method comprising: filling substantially the first tank with cold liquid and the second tank with hot gas; pressurizing the first tank with the hot gas from the second tank; flowing cold liquid into the second tank while expanding the liquid from the first tank to form a gas; supplying the gas to a prime mover; condensing the gas from the prime mover to said cold liquid; and repeating said foregoing steps by swapping said filling, said pressurizing, and said flowing steps between said first and second tanks, thereby circulating said fluids through said system. 